1. Field of Invention
The present invention relates to a laser annealing apparatus and its applications. More particularly, the present invention relates to a laser annealing apparatus that can stabilize the surrounding atmosphere of a laser-irradiated region and the application of the same.
2. Description of Related Art
With the rapid development in technologies, high tech products including video, especially digital video or imaging devices has become an indispensable part in our everyday life. At present, liquid crystal display (LCD) is one of the commonest video and image display devices. In recent years, a type of thin film transistor liquid crystal display (TFT-LCD) manufactured using polysilicon fabrication technique is produced. The TFT-LCD fabricated using polysilicon has electron mobility considerably greater than a conventional TFT-LCD device made from amorphous silicon. Consequently, a smaller thin film transistor with a larger aperture ratio can be built so that brightness level of the display is increased and power consumption of the device is reduced. Moreover, due to an increase in electron mobility, a portion of the driving circuit may be fabricated on the glass substrate along with the thin film transistors. Ultimately, reliability and performance of the liquid crystal display panel is improved and production cost of the display is reduced considerably. Due to a lower production cost of a polysilicon TFT-LCD considerable improvements in the resolution, thickness and weight of the polysilicon TFT-LCD compared with the amorphous silicon TFT-LCD, polysilicon TFT-LCD is routinely used in mobile and portable products to take advantage of its light weight and low power consumption.
Formerly, the polysilicon thin film transistors are fabricated through solid phase crystallization (SPC). However, to withstand the high re-crystallization temperature of up to 1000Â° C., quartz substrate must be used. Yet, quartz substrate costs considerably more than a glass substrate. Moreover, due to limitations in the size of substrate, a panel having a dimension of at most 2 to 3 inches can be produced. Hence, only small polysilicon display panels are produced. With the advent of laser technologies, the transformation of amorphous silicon film into polysilicon film is carried out through laser irradiation. For example, an amorphous silicon film is scanned by a laser source in a laser crystallization or excimer laser annealing (ELA) process so that the amorphous silicon re-crystallizes into polysilicon at a temperature around 600Â° C. Since a lower temperature is used to transform the silicon, ordinary glass panel used in most conventional amorphous silicon TFT can be used to fabricate the polysilicon TFT. Thus, the fabrication of a polysilicon TFT display panel having a larger size is possible. This type of low temperature re-crystallization technique of manufacturing polysilicon is often referred to as a low temperature polysilicon (LTPS) process.
FIG. 1 is a schematic diagram of a conventional laser annealing apparatus for transforming amorphous silicon film into polysilicon film. As shown in FIG. 1, a laser annealing apparatus 100 principally includes a laser source 102 and a chamber 104. An incoming laser window 106 made from quartz glass is located above the chamber 104. A beam of laser 102a from the laser source 102 penetrates the laser window 106 and irradiates the interior of the chamber 104. In addition, a movable apparatus 108 is placed inside the chamber 104. A substrate 110 is normally placed over the movable apparatus 108 so tat the substrate 110 is able to shift position when irradiated by the laser.
In general, liquefied silicon may react with moisture and oxygen in surrounding atmosphere to form contaminants when amorphous silicon film is irradiated with a laser in a re-crystallization process. To prevent contaminants produced inside the chamber 104 from contaminating the polysilicon film and resulting in some unforeseen effects, gaseous nitrogen or inert gas is passed into the chamber 104.
Nevertheless, the aforementioned method can hardly stabilize the atmosphere around a laser-illuminated region so that all kinds of unwanted gaseous contaminants may still pollute the surrounding atmosphere during the laser annealing process. The unwanted gas includes silicon vapor evaporated from the amorphous silicon film at high temperature. One of the components inside the chamber 104 that can be easily polluted by the silicon vapor is the laser window 106. With too much pollution, working life of the incoming laser window 106 may be greatly reduced. To prevent this from happening, an inert gas inlet 104a and an inert gas outlet 104b are installed on each side of the chamber 104 as shown in FIG. 1. Utilizing the purging effect of inert gases, quality of the atmosphere inside the chamber 104 may be conditioned. However, because size of the chamber 104 for carrying out laser annealing process is usually large, a lot of inert gas must be used to purge the atmosphere. Moreover, this arrangement can only produce partial improvement in the atmospheric quality. In fact, little is done to stabilize the atmosphere close to the laser-irradiated region.